getClosestKeys

Gets a List of possible keys to be used when searching for the closest
LocalizedString for a given locale/charset pair. The List starts with a
key for the specific locale (ignored if null) followed by a lookup
precedence list of alternative keys (inspired on
java.util.ResourceBundle) as shown bellow.

charset + "_" + language1 + "_" + country1 + "_" + variant1

charset + "_" + language1 + "_" + country1

charset + "_" + language1

charset + "_" + language2 + "_" + country2 + "_" + variant2

charset + "_" + language2 + "_" + country2

charset + "_" + language2

charset + "_" + language3 + "_" + country3 + "_" + variant3

charset + "_" + language3 + "_" + country3

charset + "_" + language3

Where:

Candidate keys where the final component is an empty string are omitted.
For example, if country1 is an empty string, charset + "_" + language1 +
"_" + country1 is omitted;